In general, a chair, which is used at houses, offices, working environments, etc. for a very long time, includes a chair frame and a seat member, a backrest member, and an armrest disposed at the chair frame.
Recently, as the sitting time increases, it becomes necessary to provide chairs that provide the best sitting feeling and convenience so that the sitter feels reduced fatigue even though he or she sits for a long while.
Especially, since the posture of the sitter gets disordered in cases of long-term working or studying thereon and such disordered postures have the backbone under stresses causing disc problem or bending of backbone, it is very important to guide the sitter to sit on the chair in right postures or to support the waist portion of the sitter.
Chairs with backrests bendable backwards in order to relieve fatigues of the sitter are being used widely, but in such chairs with backrests bendable backwards the portion for supporting the waist disappears when the backrest bends backwards so as to make the sitter's waist portion more uncomfortable, so that the sitter has to use such chairs with a separate cushion put between the backrest and his or her waist, such a cushion may be lost or flattened by long-term usage, which incurs the cost to replace the cushion with a new one.